The colour, and where it comes from
The turquoise everyone photographs is not spread evenly across the bay. It is shallow water over pale sand, lit from above. The darker patches beside it are seagrass, just as clear but nothing like as bright.
The line between the two is sharp enough to see from the deck. That is the whole trick: you can read the water and park on the patch that glows, instead of taking whatever sits in front of the nearest beach. Fifty metres is often the difference.
Catching it at its best
Mornings are flat, and a flat surface is what lets the colour come up through the water instead of skidding off it. By mid-afternoon the breeze has arrived and the same cove is grey by comparison. The water you have seen in photographs is a morning at anchor.
In it, not just over it
Most calas shelve from bright sand into rock and weed at the edges, which is where the fish are, and you can swim it straight off the platform. Some boats carry masks, and a Seabob is available on part of the fleet. Both are worth asking about when you enquire.
Skippers can be added to almost any boat, rates varying by boat. It means nobody in the group is watching the depth instead of the view.